The discussion following in this specification is directed to metal structures contained in an electrolyte and to method and apparatus to enable the surveying of such structures with a high level of accuracy and efficiency. In particular, such a structure will be described as a pipeline, but other structures may be surveyed. The electrolyte in which the pipeline is contained is described as sea water and/or sea bottom, but the electrolyte may be other environments, including, for example, soil that is not necessarily submerged. The survey itself is affected by measuring the electrical potential difference between the pipeline and the electrolyte existing at distance locations along the pipeline; however, it is contemplated that other types of electrical measurements may be made in connection with such surveys. The data obtained in such surveys may be used as an indication of the condition of the pipeline, the probability that a corrosion problem will occur at a given location on the pipeline, the effectiveness of cathodic protection apparatus associated with the pipeline, etc.
In view of the very corrosive environment in which offshore pipelines are installed, cathodic protection is generally required. Monitoring of the level of cathodic protection is an essential component of maintaining effective corrosion control; data from surveys in which a series of electrical measurements are taken indicate whether the installed cathodic protection equipment is functioning properly. The surveys may be made of pipelines with galvanic cathodic protection or impressed current cathodic protection.
In the past, to examine or to determine the level or effectiveness of cathodic protection of an onshore buried pipeline, pipe-to-soil potentials were measured at intervals along the length of the line. The intervals were selected to provide a representative profile of the potentials along the entire length of the line. One example of an apparatus and method for making such a survey is presented in U.S. Pat. No. 4,151,458, which is assigned to the assignee of the present application.
Another example of apparatus and method of the computerized type for making such surveys, either onshore or offshore, is presented in U.S. patent application Ser. No. 17,180, filed Mar. 5, 1979, now U.S. Pat. No. 4,322,805 which application is assigned to the assignee of the present application. Still another example of apparatus and method directed particularly to an offshore pipeline electrical survey is disclosed in U.S. patent application Ser. No. 881,737, filed Feb. 27, 1978, now U.S. Pat. No. 4,228,399 which also is assigned to the assignee of the present application.
The disclosures of the above-mentioned patent and applications are hereby incorporated by reference. Also, as used here marine and offshore are equivalent and interchangeable.
Very generally, the survey techniques disclosed in such patent and applications broadly include the connecting of a wire to a test station, which is electrically connected to the pipeline, moving an electrode to certain locations along the pipeline to make contact with the electrolyte in which the pipeline is contained, measuring the electrical potential difference between the electrolyte via the electrode and the pipeline via the wire, and correlating the potential difference data with the distance locations along the pipeline at which such electrical measurements are made and data taken.
It will be appreciated that the task of acquiring a complete potential profile on an offshore pipeline may be more difficult than is the case for an onshore pipeline. In offshore surveys the distance between test stations frequently exceeds 20 miles; whereas adjacent test stations for onshore pipelines ordinarily are much closer. Thus, the strength of the wire used in an offshore survey must be adequate to avoid breakage when streamed out over the relatively long distances encountered, and the accuracy of data collecting and measuring and of distance measuring must be relatively high. Moreover, preferably the wire should be reasonably light weight and gauge for optimum facility of usage and storage and for minimum cost. Additionally, the insulation on the wire used in offshore surveys should have a minimum of holidays or areas in the insulation to which water may find easy access to the conductor which would have a detrimental effect on potential difference measurement; on the other hand, in onshore surveys the criticality of insulation integrity is not as severe. Furthermore, in the offshore survey the wire should be economically disposable and should be of a size, weight and strength such that even if the wire were dropped or disposed of in place at the conclusion of a survey without picking up the wire, such in place disposal would not have a significant effect on the environment.
It is often desirable to provide electrical insulation between that portion of a pipeline ordinarily submerged and that portion extending onshore or onto a platform to prevent stray signals from entering the submerged pipeline and to optimize the effectiveness of the cathodic protection system for the pipeline. However, sometimes the effectiveness of an insulating member in the pipeline may be reduced or totally lost, e.g. short circuited, and, therefore, it is desirable to enable a facile technique for examining the effectiveness of such insulating members. In other instances line currents, i.e. currents flowing in the pipeline, are intended, but such line currents may detrimentally affect the accuracy of potential difference measurements and the complete potential profile of a pipeline. Thus, it would be desirable to correct data taken in such surveys to eliminate or to overcome the effect of line currents on such measurements.
Another difficulty in offshore surveys is maintaining accurate distance information, vis-a-vis the locations along the pipeline at which potential difference measurements are taken. The wire may drift, for it is undesirable to maintain tight tension on the wire which might unnecessarily cause a breakage. With the wire being used not only for electrical connecting purposes but also for distance measuring purposes, for example as is disclosed in the above mentioned patent and application, such drifting and/or sinking of the wire will cause inaccuracies in the distance measurement data; such inaccuracies generally are avoided in onshore surveys because there is no drifting or sinking and due to the relatively close proximity of adjacent test stations. Thus, it would be desirable to maintain the accuracy of distance information obtained in offshore surveys.